Method for performing a buffer status reporting in a D2D communication system and device therefor

ABSTRACT

The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for performing a buffer status reporting in a D2D communication system, the method comprising: configuring a plurality of sidelink logical channels, wherein each of the plurality of sidelink logical channels has an associated priority and belongs to a LCG, and one or more of LCGs belong to a ProSe Destination; generating a SL BSR MAC CE including buffer sizes of LCGs in decreasing order of a LCG priority, wherein the LCG priority of a LCG is determined as a highest priority of a logical channel among priorities of logical channels belonging to the LCG; and transmitting a MAC PDU including the SL BSR MAC CE.

Pursuant to U.S.C. § 119(e), this application claims the benefit of U.S. Provisional Application No. 62/204,410, filed on Aug. 12, 2015, the contents of which are all hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a wireless communication system and, more particularly, to a method for performing a buffer status reporting in a D2D (Device to Device) communication system and a device therefor.

Discussion of the Related Art

As an example of a mobile communication system to which the present invention is applicable, a 3rd Generation Partnership Project Long Term Evolution (hereinafter, referred to as LTE) communication system is described in brief.

FIG. 1 is a view schematically illustrating a network structure of an E-UMTS as an exemplary radio communication system. An Evolved Universal Mobile Telecommunications System (E-UMTS) is an advanced version of a conventional Universal Mobile Telecommunications System (UMTS) and basic standardization thereof is currently underway in the 3GPP. E-UMTS may be generally referred to as a Long Term Evolution (LTE) system. Details of the technical specifications of UMTS and E-UMTS are provided in Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”, for example.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs (eNBs), and an Access Gateway (AG) which is located at an end of the network (E-UTRAN) and connected to an external network. The eNBs may simultaneously transmit multiple data streams for a broadcast service, a multicast service, and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in one of bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides a downlink (DL) or uplink (UL) transmission service to a plurality of UEs in the bandwidth. Different cells may be set to provide different bandwidths. The eNB controls data transmission or reception to and from a plurality of UEs. The eNB transmits DL scheduling information of DL data to a corresponding UE so as to inform the UE of a time/frequency domain in which the DL data is supposed to be transmitted, coding, a data size, and hybrid automatic repeat and request (HARQ)-related information. In addition, the eNB transmits UL scheduling information of UL data to a corresponding UE so as to inform the UE of a time/frequency domain which may be used by the UE, coding, a data size, and HARQ-related information. An interface for transmitting user traffic or control traffic may be used between eNBs. A core network (CN) may include the AG and a network node or the like for user registration of UEs. The AG manages the mobility of a UE on a tracking area (TA) basis. One TA includes a plurality of cells.

Device to device (D2D) communication refers to the distributed communication technology that directly transfers traffic between adjacent nodes without using infrastructure such as a base station. In a D2D communication environment, each node such as a portable terminal discovers user equipment physically adjacent thereto and transmits traffic after setting communication session. In this way, since D2D communication may solve traffic overload by distributing traffic concentrated into the base station, the D2D communication may have received attention as the element technology of the next generation mobile communication technology after 4G. For this reason, standard institutes such as 3GPP or IEEE have proceeded to establish a D2D communication standard on the basis of LTE-A or Wi-Fi, and Qualcomm has developed their own D2D communication technology.

It is expected that D2D communication contributes to increase throughput of a mobile communication system and create new communication services. Also, D2D communication may support proximity based social network services or network game services. The problem of link of a user equipment located at a shade zone may be solved by using a D2D link as a relay. In this way, it is expected that the D2D technology will provide new services in various fields.

D2D communication technologies such as infrared communication, ZigBee, radio frequency identification (RFID) and near field communications (NFC) based on RFID have been already used. However, since these technologies support communication only of a specific object within a limited distance (about 1 m), it is difficult for the technologies to be strictly regarded as D2D communication technologies.

Although D2D communication has been described as above, details of a method for transmitting data from a plurality of D2D user equipments with the same resource have not been suggested.

SUMMARY OF THE INVENTION

The object of the present invention can be achieved by providing a method for User Equipment (UE) operating in a wireless communication system as set forth in the appended claims.

In another aspect of the present invention, provided herein is a communication apparatus as set forth in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention.

FIG. 1 is a diagram showing a network structure of an Evolved Universal Mobile Telecommunications System (E-UMTS) as an example of a wireless communication system;

FIG. 2A is a block diagram illustrating network structure of an evolved universal mobile telecommunication system (E-UMTS), and FIG. 2B is a block diagram depicting architecture of a typical E-UTRAN and a typical EPC;

FIG. 3 is a diagram showing a control plane and a user plane of a radio interface protocol between a UE and an E-UTRAN based on a 3rd generation partnership project (3GPP) radio access network standard;

FIG. 4 is a diagram of an example physical channel structure used in an E-UMTS system;

FIG. 5 is a block diagram of a communication apparatus according to an embodiment of the present invention;

FIG. 6 is an example of default data path for a normal communication;

FIGS. 7 and 8 are examples of data path scenarios for a proximity communication;

FIG. 9 is a conceptual diagram illustrating for a Layer 2 Structure for Sidelink;

FIG. 10A is a conceptual diagram illustrating for User-Plane protocol stack for ProSe Direct Communication, and FIG. 10B is Control-Plane protocol stack for ProSe Direct Communication;

FIG. 11 is a diagram for MAC structure overview in a UE side; and

FIG. 12 is a diagram for performing a buffer status reporting in a D2D communication system according to embodiments of the present invention;

FIGS. 13A and 13B are diagrams for SL BSR MAC CE according to embodiments of the present invention; and

FIGS. 14A to 14D are examples for performing a buffer status reporting in a D2D communication system according to embodiments of the present invention

DETAILED DESCRIPTION OF THE INVENTION

Universal mobile telecommunications system (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.

Hereinafter, structures, operations, and other features of the present invention will be readily understood from the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Embodiments described later are examples in which technical features of the present invention are applied to a 3GPP system.

Although the embodiments of the present invention are described in the context of a long term evolution (LTE) system and a LTE-advanced (LTE-A) system in the present specification, they are purely exemplary. Therefore, the embodiments of the present invention are applicable to any other communication system corresponding to the above definition. An exemplary system in which the invention disclosed herein may be implemented is a system compliant with the 3GPP specification TS 36.321 Version 12.6.0. In addition, although the embodiments of the present invention are described based on a frequency division duplex (FDD) scheme in the present specification, the embodiments of the present invention may be easily modified and applied to a half-duplex FDD (H-FDD) scheme or a time division duplex (TDD) scheme.

FIG. 2A is a block diagram illustrating network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed to provide a variety of communication services such as voice (VoIP) through IMS and packet data.

As illustrated in FIG. 2A, the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN), an Evolved Packet Core (EPC) and one or more user equipment. The E-UTRAN may include one or more evolved NodeB (eNodeB) 20, and a plurality of user equipment (UE) 10 may be located in one cell. One or more E-UTRAN mobility management entity (MME)/system architecture evolution (SAE) gateways 30 may be positioned at the end of the network and connected to an external network.

As used herein, “downlink” refers to communication from eNodeB 20 to UE 10, and “uplink” refers to communication from the UE to an eNodeB. UE 10 refers to communication equipment carried by a user and may be also referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.

FIG. 2B is a block diagram depicting architecture of a typical E-UTRAN and a typical EPC.

As illustrated in FIG. 2B, an eNodeB 20 provides end points of a user plane and a control plane to the UE 10. MME/SAE gateway 30 provides an end point of a session and mobility management function for UE 10. The eNodeB and MME/SAE gateway may be connected via an S1 interface.

The eNodeB 20 is generally a fixed station that communicates with a UE 10, and may also be referred to as a base station (BS) or an access point. One eNodeB 20 may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNodeBs 20.

The MME provides various functions including NAS signaling to eNodeBs 20, NAS signaling security, AS Security control, Inter CN node signaling for mobility between 3GPP access networks, Idle mode UE Reachability (including control and execution of paging retransmission), Tracking Area list management (for UE in idle and active mode), PDN GW and Serving GW selection, MME selection for handovers with MME change, SGSN selection for handovers to 2G or 3G 3GPP access networks, Roaming, Authentication, Bearer management functions including dedicated bearer establishment, Support for PWS (which includes ETWS and CMAS) message transmission. The SAE gateway host provides assorted functions including Per-user based packet filtering (by e.g. deep packet inspection), Lawful Interception, UE IP address allocation, Transport level packet marking in the downlink, UL and DL service level charging, gating and rate enforcement, DL rate enforcement based on APN-AMBR. For clarity MME/SAE gateway 30 will be referred to herein simply as a “gateway,” but it is understood that this entity includes both an MME and an SAE gateway.

A plurality of nodes may be connected between eNodeB 20 and gateway 30 via the S1 interface. The eNodeBs 20 may be connected to each other via an X2 interface and neighboring eNodeBs may have a meshed network structure that has the X2 interface.

As illustrated, eNodeB 20 may perform functions of selection for gateway 30, routing toward the gateway during a Radio Resource Control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of Broadcast Channel (BCCH) information, dynamic allocation of resources to UEs 10 in both uplink and downlink, configuration and provisioning of eNodeB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 may perform functions of paging origination, LTE-IDLE state management, ciphering of the user plane, System Architecture Evolution (SAE) bearer control, and ciphering and integrity protection of Non-Access Stratum (NAS) signaling.

The EPC includes a mobility management entity (MME), a serving-gateway (S-GW), and a packet data network-gateway (PDN-GW). The MME has information about connections and capabilities of UEs, mainly for use in managing the mobility of the UEs. The S-GW is a gateway having the E-UTRAN as an end point, and the PDN-GW is a gateway having a packet data network (PDN) as an end point.

FIG. 3 is a diagram showing a control plane and a user plane of a radio interface protocol between a UE and an E-UTRAN based on a 3GPP radio access network standard. The control plane refers to a path used for transmitting control messages used for managing a call between the UE and the E-UTRAN. The user plane refers to a path used for transmitting data generated in an application layer, e.g., voice data or Internet packet data.

A physical (PHY) layer of a first layer provides an information transfer service to a higher layer using a physical channel. The PHY layer is connected to a medium access control (MAC) layer located on the higher layer via a transport channel. Data is transported between the MAC layer and the PHY layer via the transport channel. Data is transported between a physical layer of a transmitting side and a physical layer of a receiving side via physical channels. The physical channels use time and frequency as radio resources. In detail, the physical channel is modulated using an orthogonal frequency division multiple access (OFDMA) scheme in downlink and is modulated using a single carrier frequency division multiple access (SC-FDMA) scheme in uplink.

The MAC layer of a second layer provides a service to a radio link control (RLC) layer of a higher layer via a logical channel. The RLC layer of the second layer supports reliable data transmission. A function of the RLC layer may be implemented by a functional block of the MAC layer. A packet data convergence protocol (PDCP) layer of the second layer performs a header compression function to reduce unnecessary control information for efficient transmission of an Internet protocol (IP) packet such as an IP version 4 (IPv4) packet or an IP version 6 (IPv6) packet in a radio interface having a relatively small bandwidth.

A radio resource control (RRC) layer located at the bottom of a third layer is defined only in the control plane. The RRC layer controls logical channels, transport channels, and physical channels in relation to configuration, re-configuration, and release of radio bearers (RBs). An RB refers to a service that the second layer provides for data transmission between the UE and the E-UTRAN. To this end, the RRC layer of the UE and the RRC layer of the E-UTRAN exchange RRC messages with each other.

One cell of the eNB is set to operate in one of bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides a downlink or uplink transmission service to a plurality of UEs in the bandwidth. Different cells may be set to provide different bandwidths.

Downlink transport channels for transmission of data from the E-UTRAN to the UE include a broadcast channel (BCH) for transmission of system information, a paging channel (PCH) for transmission of paging messages, and a downlink shared channel (SCH) for transmission of user traffic or control messages. Traffic or control messages of a downlink multicast or broadcast service may be transmitted through the downlink SCH and may also be transmitted through a separate downlink multicast channel (MCH).

Uplink transport channels for transmission of data from the UE to the E-UTRAN include a random access channel (RACH) for transmission of initial control messages and an uplink SCH for transmission of user traffic or control messages. Logical channels that are defined above the transport channels and mapped to the transport channels include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic channel (MTCH).

FIG. 4 is a view showing an example of a physical channel structure used in an E-UMTS system. A physical channel includes several subframes on a time axis and several subcarriers on a frequency axis. Here, one subframe includes a plurality of symbols on the time axis. One subframe includes a plurality of resource blocks and one resource block includes a plurality of symbols and a plurality of subcarriers. In addition, each subframe may use certain subcarriers of certain symbols (e.g., a first symbol) of a subframe for a physical downlink control channel (PDCCH), that is, an L1/L2 control channel. In FIG. 4, an L1/L2 control information transmission area (PDCCH) and a data area (PDSCH) are shown. In one embodiment, a radio frame of 10 ms is used and one radio frame includes 10 subframes. In addition, one subframe includes two consecutive slots. The length of one slot may be 0.5 ms. In addition, one subframe includes a plurality of OFDM symbols and a portion (e.g., a first symbol) of the plurality of OFDM symbols may be used for transmitting the L1/L2 control information. A transmission time interval (TTI) which is a unit time for transmitting data is 1 ms.

A base station and a UE mostly transmit/receive data via a PDSCH, which is a physical channel, using a DL-SCH which is a transmission channel, except a certain control signal or certain service data. Information indicating to which UE (one or a plurality of UEs) PDSCH data is transmitted and how the UE receive and decode PDSCH data is transmitted in a state of being included in the PDCCH.

For example, in one embodiment, a certain PDCCH is CRC-masked with a radio network temporary identity (RNTI) “A” and information about data is transmitted using a radio resource “B” (e.g., a frequency location) and transmission format information “C” (e.g., a transmission block size, modulation, coding information or the like) via a certain subframe. Then, one or more UEs located in a cell monitor the PDCCH using its RNTI information. And, a specific UE with RNTI “A” reads the PDCCH and then receive the PDSCH indicated by B and C in the PDCCH information.

FIG. 5 is a block diagram of a communication apparatus according to an embodiment of the present invention.

The apparatus shown in FIG. 5 can be a user equipment (UE) and/or eNB adapted to perform the above mechanism, but it can be any apparatus for performing the same operation.

As shown in FIG. 5, the apparatus may comprises a DSP/microprocessor (110) and RF module (transmiceiver; 135). The DSP/microprocessor (110) is electrically connected with the transciver (135) and controls it. The apparatus may further include power management module (105), battery (155), display (115), keypad (120), SIM card (125), memory device (130), speaker (145) and input device (150), based on its implementation and designer's choice.

Specifically, FIG. 5 may represent a UE comprising a receiver (135) configured to receive a request message from a network, and a transmitter (135) configured to transmit the transmission or reception timing information to the network. These receiver and the transmitter can constitute the transceiver (135). The UE further comprises a processor (110) connected to the transceiver (135: receiver and transmitter).

Also, FIG. 5 may represent a network apparatus comprising a transmitter (135) configured to transmit a request message to a UE and a receiver (135) configured to receive the transmission or reception timing information from the UE. These transmitter and receiver may constitute the transceiver (135). The network further comprises a processor (110) connected to the transmitter and the receiver. This processor (110) may be configured to calculate latency based on the transmission or reception timing information.

Recently, Proximity-based Service (ProSe) has been discussed in 3GPP. The ProSe enables different UEs to be connected (directly) each other (after appropriate procedure(s), such as authentication), through eNB only (but not further through Serving Gateway (SGW)/Packet Data Network Gateway (PDN-GW, PGW)), or through SGW/PGW. Thus, using the ProSe, device to device direct communication can be provided, and it is expected that every devices will be connected with ubiquitous connectivity. Direct communication between devices in a near distance can lessen the load of network. Recently, proximity-based social network services have come to public attention, and new kinds of proximity-based applications can be emerged and may create new business market and revenue. For the first step, public safety and critical communication are required in the market. Group communication is also one of key components in the public safety system. Required functionalities are: Discovery based on proximity, Direct path communication, and Management of group communications.

Use cases and scenarios are for example: i) Commercial/social use, ii) Network offloading, iii) Public Safety, iv) Integration of current infrastructure services, to assure the consistency of the user experience including reachability and mobility aspects, and v) Public Safety, in case of absence of EUTRAN coverage (subject to regional regulation and operator policy, and limited to specific public-safety designated frequency bands and terminals).

FIG. 6 is an example of default data path for communication between two UEs. With reference to FIG. 6, even when two UEs (e.g., UE1, UE2) in close proximity communicate with each other, their data path (user plane) goes via the operator network. Thus a typical data path for the communication involves eNB(s) and/or Gateway(s) (GW(s)) (e.g., SGW/PGW).

FIGS. 7 and 8 are examples of data path scenarios for a proximity communication. If wireless devices (e.g., UE1, UE2) are in proximity of each other, they may be able to use a direct mode data path (FIG. 7) or a locally routed data path (FIG. 8). In the direct mode data path, wireless devices are connected directly each other (after appropriate procedure(s), such as authentication), without eNB and SGW/PGW. In the locally routed data path, wireless devices are connected each other through eNB only.

FIG. 9 is a conceptual diagram illustrating for a Layer 2 structure for Sidelink.

Sidelink communication is a mode of communication whereby UEs can communicate with each other directly over the PC5 interface This communication mode is supported when the UE is served by E-UTRAN and when the UE is outside of E-UTRA coverage. Only those UEs authorized to be used for public safety operation can perform sidelink communication.

In order to perform synchronization for out of coverage operation UE(s) may act as a synchronization source by transmitting SBCCH and a synchronization signal. SBCCH carries the most essential system information needed to receive other sidelink channels and signals. SBCCH along with a synchronization signal is transmitted with a fixed periodicity of 40 ms. When the UE is in network coverage, the contents of SBCCH are derived from the parameters signalled by the eNB. When the UE is out of coverage, if the UE selects another UE as a synchronization reference, then the content of SBCCH is derived from the received SBCCH; otherwise UE uses pre-configured parameters. SIB18 provides the resource information for synchronization signal and SBCCH transmission. There are two pre-configured subframes every 40 ms for out of coverage operation. UE receives synchronization signal and SBCCH in one subframe and transmit synchronization signal and SBCCH on another subframe if UE becomes synchronization source based on defined criterion.

UE performs sidelink communication on subframes defined over the duration of Sidelink Control period. The sidelink Control period is the period over which resources allocated in a cell for sidelink control information and sidelink data transmissions occur. Within the sidelink Control period the UE sends sidelink control information followed by sidelink data. sidelink control information indicates a Layer 1 ID and characteristics of the transmissions (e.g. MCS, location of the resource(s) over the duration of Sidelink Control period, timing alignment).

The UE performs transmission and reception over Uu and PC5 with the following decreasing priority order:

-   -   Uu transmission/reception (highest priority);     -   PC5 sidelink communication transmission/reception;     -   PC5 sidelink discovery announcement/monitoring (lowest         priority).

FIG. 10A is a conceptual diagram illustrating for User-Plane protocol stack for ProSe Direct Communication, and FIG. 10B is Control-Plane protocol stack for ProSe Direct Communication.

FIG. 10A shows the protocol stack for the user plane, where PDCP, RLC and MAC sublayers (terminate at the other UE) perform the functions listed for the user plane (e.g. header compression, HARQ retransmissions). The PC5 interface consists of PDCP, RLC, MAC and PHY as shown in FIG. 10A.

User plane details of ProSe Direct Communication: i) there is no HARQ feedback for sidelink communication, ii) RLC UM is used for sidelink communication, iii) RLC UM is used for sidelink communication, iv) a receiving RLC UM entity used for sidelink communication does not need to be configured prior to reception of the first RLC UMD PDU, and v) ROHC Unidirectional Mode is used for header compression in PDCP for sidelink communication.

A UE may establish multiple logical channels. LCID included within the MAC subheader uniquely identifies a logical channel within the scope of one Source Layer-2 ID and ProSe Layer-2 Group ID combination. Parameters for logical channel prioritization are not configured. The Access stratum (AS) is provided with the PPPP of protocol data unit transmitted over PC5 interface by higher layer. There is a PPPP associated with each logical channel.

SL-RNTI is an unique identification used for ProSe Direct Communication Scheduling.

The Source Layer-2 ID identifies the sender of the data in sidelink communication. The Source Layer-2 ID is 24 bits long and is used together with Destination Layer-2 ID and LCID for identification of the RLC UM entity and PDCP entity in the receiver.

The destination Layer-2 ID identifies the target of the data in sidelink communication. The Destination Layer-2 ID is 24 bits long and is split in the MAC layer into two bit strings: i) One bit string is the LSB part (8 bits) of Destination Layer-2 ID and forwarded to physical layer as Group Destination ID. This identifies the target of the intended data in sidelink control information and is used for filtering of packets at the physical layer. And ii) Second bit string is the MSB part (16 bits) of the Destination Layer-2 ID and is carried within the MAC header. This is used for filtering of packets at the MAC layer.

No Access Stratum signalling is required for group formation and to configure Source Layer-2 ID, Destination Layer-2 ID and Group Destination ID in the UE. These identities are either provided by higher layer or derived from identities provided by higher layer. In case of groupcast and broadcast, the ProSe UE ID provided by higher layer is used directly as the Source Layer-2 ID and the ProSe Layer-2 Group ID provided by higher layer is used directly as the Destination Layer-2 ID in the MAC layer. In case of one-to-one communications, higher layer provides Source Layer-2 ID and Destination Layer-2 ID.

FIG. 10B shows the protocol stack for the control plane.

A UE does not establish and maintain a logical connection to receiving UEs prior to one-to-many a sidelink communication. Higher layer establish and maintain a logical connection for one-to-one sidelink communication including ProSe UE-to-Network Relay operation.

The Access Stratum protocol stack for SBCCH in the PC5 interface consists of RRC, RLC, MAC and PHY as shown below in FIG. 10B.

The PPPP is a ProSe Per-Packet Priority. The ProSe Per-Packet Priority is summarized as follows:

i) a single UE shall be able to transmit packets of different priorities on PC5, ii) the UE upper layers provide to the access stratum a ProSe Per Packet Priority from a range of possible values, iii) the ProSe Per Packet Priority is used to support preferential transmission of packets both intra-UE and across different UEs, iv) the support of 8 priority levels for the ProSe Per Packet Priority should be sufficient, v) the ProSe Per Packet Priority applies to all PC5 traffic, and vi) the ProSe Per Packet Priority is independent of the layer-2 destination of the transmission.

From the above summary, it seems that SA2 think ProSe packet prioritization based on PPP is very important and should be supported in PC5 interface in any case. Keeping this observation in mind, we explain how the LCP procedures should be changed from Rel-12.

FIG. 11 is a diagram for MAC structure overview in a UE side.

The MAC layer handles logical-channel multiplexing, hybrid-ARQ retransmissions, and uplink and downlink scheduling. It is also responsible for multiplexing/demultiplexing data across multiple component carriers when carrier aggregation is used.

The MAC provides services to the RLC in the form of logical channels. A logical channel is defined by the type of information it carries and is generally classified as a control channel, used for transmission of control and configuration information necessary for operating an LTE system, or as a traffic channel, used for the user data. The set of logical-channel types specified for LTE includes:

-   -   The Broadcast Control Channel (BCCH), used for transmission of         system information from the network to all terminals in a cell.         Prior to accessing the system, a terminal needs to acquire the         system information to find out how the system is configured and,         in general, how to behave properly within a cell.     -   The Paging Control Channel (PCCH), used for paging of terminals         whose location on a cell level is not known to the network. The         paging message therefore needs to be transmitted in multiple         cells.     -   The Common Control Channel (CCCH), used for transmission of         control information in conjunction with random access.     -   The Dedicated Control Channel (DCCH), used for transmission of         control information to/from a terminal. This channel is used for         individual configuration of terminals such as different handover         messages.     -   The Multicast Control Channel (MCCH), used for transmission of         control information required for reception of the MTCH.     -   The Dedicated Traffic Channel (DTCH), used for transmission of         user data to/from a terminal. This is the logical channel type         used for transmission of all uplink and non-MBSFN downlink user         data.     -   The Multicast Traffic Channel (MTCH), used for downlink         transmission of MBMS services.

To support priority handling, multiple logical channels, where each logical channel has its own RLC entity, can be multiplexed into one transport channel by the MAC layer. At the receiver, the MAC layer handles the corresponding demultiplexing and forwards the RLC PDUs to their respective RLC entity for in-sequence delivery and the other functions handled by the RLC. To support the demultiplexing at the receiver, a MAC is used. To each RLC PDU, there is an associated sub-header in the MAC header. The sub-header contains the identity of the logical channel (LCID) from which the RLC PDU originated and the length of the PDU in bytes. There is also a flag indicating whether this is the last sub-header or not. One or several RLC PDUs, together with the MAC header and, if necessary, padding to meet the scheduled transport-block size, form one transport block which is forwarded to the physical layer.

In addition to multiplexing of different logical channels, the MAC layer can also insert the so-called MAC control elements into the transport blocks to be transmitted over the transport channels. A MAC control element is used for inband control signaling—for example, timing-advance commands and random-access response. Control elements are identified with reserved values in the LCID field, where the LCID value indicates the type of control information. Furthermore, the length field in the sub-header is removed for control elements with a fixed length.

The MAC multiplexing functionality is also responsible for handling of multiple component carriers in the case of carrier aggregation. The basic principle for carrier aggregation is independent processing of the component carriers in the physical layer, including control signaling, scheduling and hybrid-ARQ retransmissions, while carrier aggregation is invisible to RLC and PDCP. Carrier aggregation is therefore mainly seen in the MAC layer, where logical channels, including any MAC control elements, are multiplexed to form one (two in the case of spatial multiplexing) transport block(s) per component carrier with each component carrier having its own hybrid-ARQ entity.

Meanwhile, UEs that already have a valid grant obviously do not need to request uplink resources. However, to allow the scheduler to determine the amount of resources to grant to each terminal in future subframes, information about the buffer situation and the power availability is useful, as discussed above. This information is provided to the scheduler as part of the uplink transmission through MAC control element. The LCID field in one of the MAC subheaders is set to a reserved value indicating the presence of a buffer status report.

From a scheduling perspective, buffer information for each logical channel is beneficial, although this could result in a significant overhead. Logical channels are therefore grouped into logical-channel groups and the reporting is done per group. The buffer-size field in a buffer-status report indicates the amount of data available transmission across all logical channels in a logical-channel group.

The Buffer Status Reporting (BSR) procedure is used to provide a serving eNB with information about the amount of data available for transmission (DAT) in the UL buffers of the UE. RRC may control BSR reporting by configuring the three timers periodicBSR-Timer and retxBSR-Timer and logicalChannelSR-ProhibitTimer and by, for each logical channel, optionally signaling Logical Channel Group (LCG) which allocates the logical channel to an LCG.

The sidelink Buffer Status reporting procedure is used to provide the serving eNB with information about the amount of sidelink data available for transmission in the SL buffers associated with the MAC entity. RRC controls BSR reporting for the sidelink by configuring the two timers periodic-BSR-TimerSL and retx-BSR-TimerSL. Each sidelink logical channel belongs to a ProSe Destination. Each sidelink logical channel is allocated to an LCG depending on the priority of the sidelink logical channel and the mapping between LCG ID and priority which is provided by upper layers in logicalChGroupInfoList. LCG is defined per ProSe Destination.

A sidelink Buffer Status Report (BSR) shall be triggered if any of the following events occur: if the MAC entity has a configured SL-RNTI i) SL data, for a sidelink logical channel of a ProSe Destination, becomes available for transmission in the RLC entity or in the PDCP entity and either the data belongs to a sidelink logical channel with higher priority than the priorities of the sidelink logical channels which belong to any LCG belonging to the same ProSe Destination and for which data is already available for transmission, or there is currently no data available for transmission for any of the sidelink logical channels belonging to the same ProSe Destination, in which case the Sidelink BSR is referred below to as “Regular Sidelink BSR”, ii) UL resources are allocated and number of padding bits remaining after a Padding BSR has been triggered is equal to or larger than the size of the Sidelink BSR MAC control element containing the buffer status for at least one LCG of a ProSe Destination plus its subheader, in which case the Sidelink BSR is referred below to as “Padding Sidelink BSR”, iii) retx-BSR-TimerSL expires and the MAC entity has data available for transmission for any of the sidelink logical channels, in which case the Sidelink BSR is referred below to as “Regular Sidelink BSR”, iv) periodic-BSR-TimerSL expires, in which case the Sidelink BSR is referred below to as “Periodic Sidelink BSR”. Else, An SL-RNTI is configured by upper layers and SL data is available for transmission in the RLC entity or in the PDCP entity, in which case the Sidelink BSR is referred below to as “Regular Sidelink BSR”.

For Regular and Periodic Sidelink BSR, if the number of bits in the UL grant is equal to or larger than the size of a Sidelink BSR containing buffer status for all LCGs having data available for transmission plus its subheader, the MAC entity reports Sidelink BSR containing buffer status for all LCGs having data available for transmission. Else, the MAC entity reports Truncated Sidelink BSR containing buffer status for as many LCGs having data available for transmission as possible, taking the number of bits in the UL grant into consideration.

If the Buffer Status reporting procedure determines that at least one Sidelink BSR has been triggered and not cancelled: if the MAC entity has UL resources allocated for new transmission for this TTI and the allocated UL resources can accommodate a Sidelink BSR MAC control element plus its subheader as a result of logical channel prioritization, the MAC entity instructs the Multiplexing and Assembly procedure to generate the Sidelink BSR MAC control element(s), starts or restarts periodic-BSR-TimerSL except when all the generated Sidelink BSRs are Truncated Sidelink BSRs, and starts or restarts retx-BSR-TimerSL.

Else if a Regular Sidelink BSR has been triggered, if an uplink grant is not configured, a Scheduling Request shall be triggered.

A MAC PDU shall contain at most one Sidelink BSR MAC control element, even when multiple events trigger a Sidelink BSR by the time a Sidelink BSR can be transmitted in which case the Regular Sidelink BSR and the Periodic Sidelink BSR shall have precedence over the padding Sidelink BSR.

The MAC entity shall restart retx-BSR-TimerSL upon reception of an SL grant.

All triggered regular Sidelink BSRs shall be cancelled in case the remaining configured SL grant(s) valid for this SC Period can accommodate all pending data available for transmission. All triggered Sidelink BSRs shall be cancelled in case the MAC entity has no data available for transmission for any of the sidelink logical channels. All triggered Sidelink BSRs shall be cancelled when a Sidelink BSR (except for Truncated Sidelink BSR) is included in a MAC PDU for transmission. All triggered Sidelink BSRs shall be cancelled, and retx-BSR-TimerSL and periodic-BSR-TimerSL shall be stopped, when upper layers configure autonomous resource selection.

The MAC entity shall transmit at most one Regular/Periodic Sidelink BSR in a TTI. If the MAC entity is requested to transmit multiple MAC PDUs in a TTI, it may include a padding Sidelink BSR in any of the MAC PDUs which do not contain a Regular/Periodic Sidelink BSR.

All Sidelink BSRs transmitted in a TTI always reflect the buffer status after all MAC PDUs have been built for this TTI. Each LCG shall report at the most one buffer status value per TTI and this value shall be reported in all Sidelink BSRs reporting buffer status for this LCG.

In Rel-13, ProSe Per Packet Priority is defined for every packet, which is used as the mechanism to perform prioritization for ProSe Communication transmissions.

Currently, in MAC, SL BSR is used to request sidelink resource in Mode 1 operation of ProSe. When reporting sidelink BSR, the UE reports sidelink BSR by containing as many ProSe Groups as possible. In selecting ProSe Groups to be included in the SL BSR MAC CE, the UE may or may not consider ProSe Group's priority. I.e., it is up to UE implementation of which ProSe Group's buffer size would be included in the sidelink BSR. Moreover, PPPP is not considered at all

In order to consider the ProSe Per Packet Priority (PPPP) of the packet, a new method is required to consider PPPP in BSR operation.

FIG. 12 is a diagram for performing a buffer status reporting in a D2D communication system according to embodiments of the present invention.

In this invention, when the MAC entity reports sidelink BSR to the eNB, the MAC entity includes the buffer status of as many LCG of ProSe Destination as possible by considering the highest PPP of data belonging to a LCG or a ProSe Destination.

In detail, the MAC entity compares the highest PPPP of data belonging to a LCG or a ProSe Destination and includes the buffer status of a LCG of a ProSe Destination in decreasing order of PPPP of data belonging to the LCG or the ProSe Destination.

It is assumed that the UE is transmitting data to at least one ProSe Destination. The ProSe Destination refers the ProSe Group.

The UE configures a plurality of sidelink logical channels (S1201). One of the plurality of sidelink logical channels belongs to a LCG, and one or more LCGs belong to a ProSe Destination. Each of the plurality of sidelink logical channels has an associated priority which is the PPPP (ProSe Per-Packet Priority).

The PPPP is defined per data, wherein data with different PPPs can be transmitted to one ProSe Destination. The data refers PDCP SDU. PPPP is provided by the upper layer when the UE receives the packet from the upper layer.

A radio bearer is configured per ProSe Destination and per PPPP. LCG is defined per ProSe Destination, wherein sidelink logical channel for ProSe Destination can be mapped to one of the LCGs defined for that ProSe Destinations based on PPPP.

If sidelink BSR is triggered, the UE generates a SL BSR MAC CE including buffer sizes of LCGs in decreasing order of a LCG priority (S1203).

The LCG priority of a LCG is determined as a highest priority of a logical channel among priorities of the logical channels belonging to the LCG. The Access stratum (AS) is provided with the PPPP of protocol data unit transmitted over PC5 interface by higher layer. There is a PPPP associated with each logical channel. So, LCG Priority can refer the highest PPP of data belonging to a LCG of a ProSe Destination among the PPPs of data belonging to the LCG of the ProSe Destination.

If UL grant is not enough to include SL BSR MAC CE containing buffer status (BS) of all LCG of all ProSe Destinations having data available for transmission plus its subheader, The MAC entity compares LCG Priority of LCGs having data available for transmission, and the MAC entity selects BS to be included in the as many LCGs having data available for transmission as possible in decreasing order of the LCG Priority.

If UL grant is enough to include SL BSR MAC CE containing buffer status (BS) of all LCG of all ProSe Destinations having data available for transmission plus its subheader, the MAC entity shall select all LCGs of all ProSe Destinations having data available for transmission.

The MAC entity transmits a MAC PDU including the SL BSR MAC CE (S1205).

Meanswhile, if sidelink BSR is triggered, the UE can generate a SL BSR MAC CE including buffer sizes of LCGs in decreasing order of a Group priority. The Group Priority refers the highest PPP of data belonging to a ProSe Destination among the PPPPs of data belonging to the ProSe Destination. In this case, if UL grant is not enough to include SL BSR MAC CE containing buffer status (BS) of all LCG of all ProSe Destinations having data available for transmission plus its subheader, the MAC entity comparers Group priority of ProSe Destinations having data available for transmission, and selects BS to be included in the as many ProSe Destinations having data available for transmission as possible in decreasing order of the Group Priority.

If UL grant is enough to include SL BSR MAC CE containing buffer status (BS) of all LCG of all ProSe Destinations having data available for transmission plus its subheader, the MAC entity shall select all LCGs of all ProSe Destinations having data available for transmission.

The MAC entity generates SL BSR MAC CE by containing the BS of the selected LCG of ProSe Destination in decreasing order of Group Priority. For the same ProSe Destination, the MAC entity includes BS of each LCG in decreasing order of LCG priority of the LCGs belonging to that ProSe Destination. The MAC entity transmits a MAC PDU including the SL BSR MAC CE

In summary, Buffer Sizes of LCGs are included in decreasing order of the highest priority of the sidelink logical channel belonging to the LCG irrespective of the value of the Destination Index field. That is, the UE determines the LCG priority of the LCG without considering an index of a ProSe Destination which the LCG belongs to.

FIGS. 13A and 13B are diagrams for SL BSR MAC CE according to embodiments of the present invention.

FIG. 13A is for Sidelink BSR and Truncated Sidelink BSR MAC control element for even N, and FIG. 13B is for Sidelink BSR and Truncated Sidelink BSR MAC control element for odd N.

The sidelink BSR and Truncated Sidelink BSR MAC control elements consist of one Destination Index field, one LCG ID field and one corresponding Buffer Size field per reported target group.

The Sidelink BSR MAC control elements are identified by MAC PDU subheaders with LCIDs as specified in Table 1. They have variable sizes.

TABLE 1 Index LCID values 00000 CCCH 00001-01010 Identity of the logical channel 01011 CCCH 01100-10101 Reserved 10110 Truncated Sidelink BSR 10111 Sidelink BSR 11000 Dual Connectivity Power Headroom Report 11001 Extended Power Headroom Report 11010 Power Headroom Report 11011 C-RNTI 11100 Truncated BSR 11101 Short BSR 11110 Long BSR 11111 Padding

For each included group, the fields are defined as follows:

The Destination Index field identifies the ProSe Destination. The length of this field is 4 bits. The value is set to the index of the destination reported in destinationInfoList and if destinationInfoListUC is also reported, the value is indexed sequentially across both lists.

The Logical Channel Group ID field identifies the group of logical channel(s) which buffer status is being reported. The length of the field is 2 bits.

The Buffer Size field identifies the total amount of data available across all logical channels of a LCG of a ProSe Destination after all MAC PDUs for the TTI have been built. The amount of data is indicated in number of bytes. It shall include all data that is available for transmission in the RLC layer and in the PDCP layer.

The size of the RLC and MAC headers are not considered in the buffer size computation. The length of this field is 6 bits. The values taken by the Buffer Size field are shown in Table 2.

R: Reserved bit, set to “0”.

TABLE 2 Index Buffer Size (BS) value [bytes] 0 BS = 0 1  0 < BS <= 10 2 10 < BS <= 12 3 12 < BS <= 14 4 14 < BS <− 17 5 17 < BS <= 19 6 19 < BS <= 22 7 22 < BS <= 26 8 26 < BS <= 31 9 31 < BS <= 36 10 36 < BS <− 42 11 42 < BS <= 49 12 49 < BS <= 57 13 57 < BS <= 67 14 67 < BS <= 78 15 78 < BS <= 91 16  91 < BS <= 107 17 107 < BS <= 125 18 125 < BS <= 146 19 146 < BS <= 171 20 171 < BS <= 200 21 200 < BS <= 234 22 234 < BS <= 274 23 274 < BS <= 321 24 321 < BS <= 376 25 376 < BS <= 440 26 440 < BS <= 515 27 515 < BS <= 603 28 603 < BS <= 706 29 706 < BS <= 826 30 826 < BS <− 967 31  967 < BS <= 1132 32 1132 < BS <= 1326 33 1326 < BS <= 1552 34 1552 < BS <= 1817 35 1817 < BS <= 2127 36 2127 < BS <− 2490 37 2490 < BS <= 2915 38 2915 < BS <= 3413 39 3413 < BS <= 3995 40 3995 < BS <= 4677 41 4677 < BS <= 5476 42 5476 < BS <− 6411 43 6411 < BS <= 7505 44 7505 < BS <= 8787 45  8787 < BS <= 10287 46 10287 < BS <= 12043 47 12043 < BS <= 14099 48 14099 < BS <= 16507 49 16507 < BS <= 19325 50 19325 < BS <= 22624 51 22624 < BS <= 26487 52 26487 < BS <= 31009 53 31009 < BS <= 36304 54 36304 < BS <= 42502 55 42502 < BS <= 49759 56 49759 < BS <= 58255 57 58255 < BS <= 68201 58 68201 < BS <= 79846 59 79846 < BS <= 93479 60  93479 < BS <= 109439 61 109439 < BS <= 128125 62 128125 < BS <− 150000 63    BS > 150000

FIGS. 14A to 14D are examples for performing a buffer status reporting in a D2D communication system according to embodiments of the present invention.

Regarding FIG. 14A, it is assumed that the UE is communicating with two ProSe Destinations 1 and 2. The UE is configured with 6 logical channels that LoCH 1 with PPPP 1, LoCH 2 with PPPP 2, LoCH 3 with PPPP4, LoCH 4 with PPPP5, LoCh 5 with PPPP3, and LoCh 6 with PPPP4. For ProSe Destination 1, LoCH 1 and LoCH 2 are mapped to LCG 1, and LoCH 3 is mapped to LCG 2. For ProSe Destination 2, LoCH 4 is mapped to LCG 1, and LoCH 5 and LoCH 5 are mapped to LCG2.

FIG. 14B is an example of SL BSR MAC CE for Case 1.

The case 1 is that UL grant can accommodate BS of all LCGs of all ProSe Destinations, and the MAC entity generates SL BSR MAC CE by containing the BS of the LCGs of ProSe Destination in decreasing order of the Group Priority.

The MAC entity decides to include BS of all LCGs of all ProSe Destinations.

The MAC entity compares the Group Priority having data available for transmission. For ProSe Destination 1, PPPP 1 is the highest PPPP and for ProSe Destination 2, PPPP 3 is the highest PPPP.

The MAC entity generates SL BSR MAC CE by including the BS of LCG of ProSe Destination in the following order: i) BS of ProSe Destination 1 is included first and then BS of ProSe Destination 2 is included later, ii) For ProSe Destination 1, BS of LCG1 is included first and then LCG2 is included later, and iii) For ProSe Destination 2, BS of LCG 2 is included first and then LCG 1 is included later.

FIG. 14C is an example of SL BSR MAC CE for Case 2.

The case 2 is that UL grant can accommodate BS of all LCGs of all ProSe Destinations, and the MAC entity generates SL BSR MAC CE by containing the BS of the LCGs of ProSe Destination in decreasing order of the LCG Priority.

The MAC entity decides to include BS of all LCGs of all ProSe Destinations.

The MAC entity compares the LCG Priority of LCGs having data available for transmission. For LCG 1 of ProSe Destination 1, PPPP 1 is the highest PPPP (1st highest PPPP), for LCG 2 of ProSe Destination 1, PPPP 4 is the highest PPPP) (3rd highest PPPP), for LCG 1 of ProSe Destination 2, PPPP 5 is the highest PPPP (4th highest PPPP), and for LCG 2 of ProSe Destination 2, PPPP 3 is the highest PPPP (2nd highest PPPP).

The MAC entity generates SL BSR MAC CE by including the BS of LCG of ProSe Destination in the following order: i) LCG 1 of ProSe Destination 1, ii) LCG 2 of ProSe Destination 2, iii) LCG 2 of ProSe Destination 1, and iv) LCG1 of ProSe Destination 2.

FIG. 14D is an example of SL BSR MAC CE for Case 3.

The case 3 is that UL grant cannot accommodate BS of all LCGs of all ProSe Destinations but can accommodate two BSs, and the MAC entity generates SL BSR MAC CE by containing the BS of the LCGs of ProSe Destination in decreasing order of the LCG Priority.

The MAC entity selects two BSs of LCG of ProSe Destination to be reported as follows:

The MAC entity compares the LCG Priority of LCGs having data available for transmission: for LCG 1 of ProSe Destination 1, PPPP 1 is the highest PPPP (1st highest PPPP), for LCG 2 of ProSe Destination 1, PPPP 4 is the highest PPPP (3rd highest PPPP), for LCG 1 of ProSe Destination 2, PPPP 5 is the highest PPPP (4th highest PPPP), and for LCG 2 of ProSe Destination 2, PPPP 3 is the highest PPPP (2nd highest PPPP).

The MAC entity selects the LCG 1 of ProSe Destination 1 and LCG 2 of ProSe Destination 2.

The MAC entity generates SL BSR MAC CE by including the BS of LCG of ProSe Destination in the following order: i) LCG 1 of ProSe Destination 1, and ii) LCG 2 of ProSe Destination 2.

The embodiments of the present invention described hereinbelow are combinations of elements and features of the present invention. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present invention may be constructed by combining parts of the elements and/or features. Operation orders described in embodiments of the present invention may be rearranged. Some constructions of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions of another embodiment. It is obvious to those skilled in the art that claims that are not explicitly cited in each other in the appended claims may be presented in combination as an embodiment of the present invention or included as a new claim by subsequent amendment after the application is filed.

In the embodiments of the present invention, a specific operation described as performed by the BS may be performed by an upper node of the BS. Namely, it is apparent that, in a network comprised of a plurality of network nodes including a BS, various operations performed for communication with an MS may be performed by the BS, or network nodes other than the BS. The term ‘eNB’ may be replaced with the term ‘fixed station’, ‘Node B’, ‘Base Station (BS)’, ‘access point’, etc.

The above-described embodiments may be implemented by various means, for example, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments of the present invention may be implemented by one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to the embodiments of the present invention may be implemented in the form of modules, procedures, functions, etc. performing the above-described functions or operations. Software code may be stored in a memory unit and executed by a processor. The memory unit may be located at the interior or exterior of the processor and may transmit and receive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present invention may be carried out in other specific ways than those set forth herein without departing from the scope of the present invention. The above embodiments are therefore to be construed in all aspects as illustrative and not restrictive. The scope of the invention should be determined by the appended claims, not by the above description, and all changes coming within the meaning of the appended claims are intended to be embraced therein. 

What is claimed is:
 1. A method for a user equipment (UE) operating in a wireless communication system, the method comprising: configuring a plurality of sidelink logical channels, generating a Sidelink Buffer Status Reporting Control Element (SL BSR CE) that includes buffer sizes of Logical Channel Groups (LCGs), and transmitting a Protocol Data Unit (PDU) that includes the SL BSR CE, wherein each of the plurality of sidelink logical channels belongs to a LCG among the LCGs and has an associated priority which is a ProSe Per-Packet Priority (PPPP), wherein each of the LCGs is related to a ProSe Destination, wherein the SL BSR CE includes the buffer sizes of the LCGs in decreasing order of respective LCG priorities of the LCGs, and wherein each of the LCG priorities of the LCGs is a highest PPPP among PPPPs of sidelink logical channels belonging to a corresponding LCG.
 2. The method according to claim 1, wherein the buffer sizes of the LCGs are included in the SL BSR CE in decreasing order of the each of the LCG priorities of the LCGs irrespective of a value of a Destination Index field of the ProSe Destination.
 3. The method according to claim 1, wherein the PDU is a Medium Access Control (MAC) PDU.
 4. A user equipment (UE) operating in a wireless communication system, the UE comprising: a Radio Frequency (RF) module; and a processor operably coupled with the RF module and configured to: configure a plurality of sidelink logical channels, generate a Sidelink Buffer Status Reporting Control Element (SL BSR CE) that includes buffer sizes of Logical Channel Groups (LCGs), and transmit a Protocol Data Unit (PDU) that includes the SL BSR CE, wherein each of the plurality of sidelink logical channels belongs to a LCG among the LCGs and has an associated priority which is a ProSe Per-Packet Priority (PPPP), wherein each of the LCGs is related to a ProSe Destination, wherein the SL BSR CE includes the buffer sizes of the LCGs in decreasing order of respective LCG priorities of the LCGs, and wherein each of the LCG priorities of the LCGs is a highest PPPP among PPPPs of sidelink logical channels belonging to a corresponding LCG.
 5. The UE according to claim 4, wherein the buffer sizes of the LCGs are included in the SL BSR CE in decreasing order of the each of the LCG priorities of the LCGs irrespective of a value of a Destination Index field of the ProSe destination.
 6. The UE according to claim 4, wherein the PDU is a Medium Access Control (MAC) PDU. 